The concrete water-reducing agent (also named superplasticizer) is an additive which can reduce the using amount of mixing water and increase the strength of the concrete in case of remaining the workability of concrete and the using amount of cement. The water-reducing agent can destruct the flocculated structure of the cement particles because of its surface activity, complexation, electrostatic repulsion, or steric repulsion, so that the workability and strength of the concrete is remaining in case of reducing the amount of the cement. The mechanism of the water-reducing agent is:
1) Dispersion: When the cement is mixed with water, because of the molecular attraction between cement particles, the cement slurry flocculates, so 1030% of the mixing water is enclosed in the flocculation, and loses free flow and lubrication. As a result, the fluidity is reduced. When the water-reducing agent is added, there are same electric charges (negative charges usually) on the surface of the cement particles, because of the oriented adsorption of the water-reducing agent molecule to the surface of the cement particles to generate the electrostatic repulsion which can force the particles to disperse. Thus, destruct the flocculation is destroyed and some water enclosed in the flocculation is released to flowing, so that the fluidity of the concrete increases.2) Lubrication: Because of strong polarity of its hydrophilic groups, the water-reducing agent adsorbs to the surface of the cement particles and forms a stable solvate water membrane with water molecules. The membrane has excellent lubricity, and can reduce the slide resistance between the cement particles, so that it can increase the concrete's fluidity.3) Steric repulsion: The hydrophilic branched chains consisted by polyether in the structure of the water-reducing agent molecule stretch in the solvent, and form a steric and hydrophilic adsorption layer on the surface of the cement particle which is adsorbed by the water-reducing agent. When different cement particles approach, these layers overlap, and the steric repulsion is produced between the cement particles. The more layers overlap, the more strong steric repulsion and stronger impediment to coacervation between cement particles, so that the excellent slump of the concrete remains.4) Delayed-release action of the branched chains in graft copolymer: During the producing of the water-reducing agent, such as polycarboxylic acid water-reducing agent, branched chains are grafted into the molecular chain of the v, wherein the branched chains do note play a role in steric repulsion, but also can release polycarboxylic acid, which promotes dispersion, due to the breaking of the branched chains, so that the dispersion effect of cement particles is increased, and the loss of the slump is controlled.
Based on the composition, the conventional water-reducing agent are divided into lignosulfonic water-reducing agent, polycyclic aromatic water-reducing agent and water-soluble resin sulfonate water-reducing agent, such as the aliphatic high-activity modified by water-reducing agent lignosulfonate disclosed in Patent CN101665337A, the concrete water-reducing agent which is a polycyclic aromatic sulfonate polymer disclosed in Patent CN101007719A, the aliphatic concrete water-reducing agent disclosed in Patent CN101575186A, and the amidosulphonate concrete water-reducing agent disclosed in Patent CN101723615A, and so on. Wherein, the conventional high-activity water-reducing agent mainly includes naphthalene sulphonate-formaldehyde condensation polymer (NSF) and melamine sulphonate-formaldehyde condensation polymer (MSF). They have high water reducing ratio, however, these water-reducing agents cannot be used for preparing high-performance or superior-performance concrete, because of the quickly loss of slump and bad permanency of the concrete when using these water-reducing agents.
Compared with the water-reducing agents above, polycarboxylic acid water-reducing agent, which is the combination product with graft multicomponent polymer of carboxylic and other available auxiliaries, is high-tech and has good combination property, such as high water reduction rate, significant enhancement effect, excellent workability of concrete, non-affection to elastic modulus of concrete, and low gradual loss when mild collapse occurs, therefore, the polycarboxylic acid water-reducing agent is widely used recently.
The excellent performance of polycarboxylic acid water-reducing agent depends on its specific molecular structure: there are many active groups, for example, —SO3H, —COOH, —OH, etc., at the main chain of the comb molecular structure, and the side chain is long polyoxylalkyl alkenyl group, for example, —(CH2CH2O)m-R, etc. The different groups results different effects to the dispersion of the cement, and usually it is considered that the electronegative sulfonic groups and carboxyl groups results anchoring and electrostatic repulsion when absorbed onto the surface of the cement particles. When stretch in the solvent, the long side chain has the strong steric repulsion which is promote dispersion of the cement particles more effectively and is more insensitive than electrostatic repulsion. So the polycarboxylic acid water-reducing agent has more effective dispersion effect and well adaptability to different cement.
In 1980s, polycarboxylic acid water-reducing agent was invented and used in project, and achieve the favorable economic benefits. The polycarboxylic acid water-reducing agent invented in Japan in 1980s is completely different from NSF and MSF. It can remain the high fluidity of the concrete when low additive amount, and can result in low viscosity and slump remaining when low water-cement ratio, and has great compatibility with various cements. So it is an indispensable component to high strength and fluidity concrete. As the needs of high strength and performance concrete, and the development of the theory of polymer chemistry and material molecular design, the demands of water-reducing agents will get more pressing. The research direction has turned to polycarboxylic acid water-reducing agent from NSF and MSF, and the polycarboxylic acid water-reducing agent has been an important component in the concrete.
The most using polycarboxylic acid water-reducing agent at present are: 1) Ester type water-reducing agent is that synthesized by two-steps method, wherein, methoxypolyethylene glycol (MPEG) and excessive methacrylic acid (or acrylic acid, maleic anhydride or itaconic acid) esterifies, and then at a temperature, dropping the aqueous solution of peroxide and aqueous solution of MPEG ester at the same time, with water as solvent and substrate. 2) Ether type water-reducing agent that is synthesized by unsaturated polyether as raw material, and unsaturated alcohol polyether and H2O (adding other monomer containing active double bond some times) as substrate, by the steps: adding peroxide as initiator, at a temperature, dropping aqueous solution of chain transfer agent and aqueous solution of comonomer containing active double bond together to copolymerization, preserving heat for aging in a period, and cooling down and neutralizing to obtain the product. The two water-reducing agents apply to different cements and each of them has advantages and disadvantages, and some times they must be used together. The two methods have a common point that is the necessary of the preservation heat and dropping active monomer when polymerizing, and continuously stirring during the dropping to mix the reactants uniformly.
Patent CN101215119A disclosed a method for synthesis of polycarboxylic acid water-reducing agent, wherein, water and alkylene polyether are added as substrate, the heated to reaction temperature, then peroxide initiator is added, and dropping comonomers and chain transfer agent to synthesize the high-performance polycarboxylic acid water-reducing agent. This one-step method is simpler than the two-steps method, however, it need large investment and strictly controlling of the process of dropping comonomers and chain transfer agent, and the consumption of energy is high because of the necessary of the heat preservation.
Patent CN101708974A disclosed a preparing method of polycarboxylic acid water-reducing agent, wherein, feeding nitrogen into solvent and heating to 70° C., dropping monomers solution and initiator solution simultaneously, and then heating to 85° C. to polymerization.
Patent CN101486783A disclosed a preparing method of polyether grafted water-reducing agent, wherein, heating to esterification in nitrogen atmosphere, and controlling the temperature at 80˜110° C. when copolymerizing, then heating to 120˜160° C. and adding sulfonating agent to sulfonattion at a high temperature. However, it is difficult to control the reaction because of excessive steps and the requirement of all the reactions at high temperature.
Patent CN101492518A disclosed a technique for preparing ether type water-reducing agent. At 60˜120° C., dropping initiator and the mixed solution of acrylic acid and 2-hydroxyethyl acrylate, then preserving heat to react.
Patent CN101024566A disclosed a technique for preparing water-reducing agent. Polyethylene glycol monomethyl ether, methacrylic acid, toluene-p-sulfonic acid and p-benzenediol are added at 80˜140° C. to react for 4˜6 hours, and then adding allyl sulfonate at 60° C., and adding initiator to polymerization at 65˜95° C. The process of this method is more complex and it is difficult to control the react temperature.
Patent CN101050083A disclosed a preparing method of polycarboxylic acid water-reducing agent r. The air in reactor is replaced by inert gas, H2O and allyl sulfonate are added. Heating to 55˜80° C. in water bath, and dropping initiator and the mixed solution of monomers simultaneously. The process of the dropping remains for 1˜2 hours. Heating to 80˜90° C. to react.
The methods above have some flaws such as large investment of producing equipment, specialized producing equipment, complex process, high energy consumption, tremendous requirement of manpower resources, etc. It will be save tremendous manpower and material resources if the technique of polymerization is improved to simplify the steps. Furthermore, there will be a long distance between construction sites in many major engineering, such as motorway or railway construction, because of the frequent transfer of construction site during the construction. It must increase the transportation cost to transport water-reducing agent (containing more than 60% H2O in the product) that produced in stationary factory to the construction site. Although, some factories can be built along the line of construction, but it will be a large investment, and the construction may be completed before the building of the factories. If the water-reducing agent is produced at the construction site with simple technique and equipment, by the raw material transported to the site, it will be decrease the investment and save transfer and producing cost.